Seroprevalence of Infectious Bronchitis Virus Antibodies in Vaccinated Broilers from South-Western Romania (2018-2021): An ELISA-Based Survey.

Infectious bronchitis virus (IBV) is an avian coronavirus associated with respiratory, renal, and reproductive disease in chickens, with important economic consequences in intensive poultry production. This study aimed to estimate the seroprevalence of anti-IBV antibodies in vaccinated broiler flocks reared in south-western Romania and to characterize its distribution across counties and study years. Between 2018 and 2021, a total of 2466 blood samples were collected from Ross 308 broilers aged 35-45 days originating from five commercial farms (one per county) located in Caraș-Severin, Dolj, Gorj, Hunedoara, and Vâlcea. Samples were obtained from 137 production halls/series. Sera were tested using a commercial indirect ELISA kit and classified according to the manufacturer's criteria based on the sample-to-positive (S/P) ratio and the corresponding antibody titer threshold. Overall, 2115/2466 sera were positive (85.77%) and 351/2466 were negative (14.23%). Anti-IBV antibodies were detected in all halls (137/137, 100%), although negative sera were recorded in 87/137 halls (63%). Seroprevalence by county ranged from 78.67% (Vâlcea) to 89.24% (Hunedoara). Significant differences in the proportions of positive and negative sera were identified between several county pairs by Fisher's exact test. These findings indicate widespread serological evidence of anti-IBV antibodies in vaccinated broiler flocks from south-western Romania, which may reflect vaccine-induced immunity and/or field exposure. The results support the need for continued serological monitoring, alongside targeted molecular investigations to differentiate vaccine strains from circulating field variants.